Title: Water quality on recirculating aquaculture systems for Arctic charr (Salvelinus alpinus L.) culture

Recirculating aquaculture systems (RAS) for fish culture have been used for more than three decades. The interest in RAS is due to their advantages such as greatly reduced land and water requirements in places where water resources are limited; but RAS also have disadvantages like the deterioration of the water quality if the water treatment processes within the system are not controlled properly. The water quality problems in RAS are associated with low dissolved oxygen (DO) and high fish waste metabolite levels in the culture water. The objective of this study is to compare water quality in a RAS with water quality in a limited reuse system (LRS) for Arctic charr culture taking into account the oxygen demands of the fish, the metabolites production by the fish, the removal of CO2 by the aerators, the removal of ammonia by the biofilter and the removal of waste products in the reused water. The experiment was conducted in Verid, the Aquaculture Research Facilities of Holar University College, Iceland, during 4 weeks. The two different systems were compared during the experiment: a RAS with a biofilter and a LRS. The results of this study showed that the water quality parameters in both systems were well within the acceptable levels for Arctic charr culture and the water quality was better in the LRS than in the RAS; the important role of the biofilter unit in the RAS was demonstrated and the necessity to control all the water treatment processes within the system, especially when the RAS is using sand filters as one of the water treatment components of the system.